1. Field of the Invention
This invention relates to a process for solvent dewaxing waxy oils. More particularly, this invention relates to a continuous, combination non-autorefrigerant/autorefrigerant solvent dewaxing process employing two chilling zones wherein a majority of the wax is precipitated in a first chilling zone in the presence of a non-autorefrigerant dewaxing solvent to form a waxy slurry which is then fed directly to a second chilling zone comprising a vertical, staged tower operating continuously at essentially constant pressure. In the second chilling zone the slurry is cooled down to wax filtration temperature and additional wax is precipitated from the oil by contact with a liquid autorefrigerant injected into a plurality of said stages, said liquid autorefrigerant evaporating in each of said stages so as to maintain an average slurry cooling rate of from 0.1.degree. to 20.degree. F. per minute and an average temperature drop per stage of from about 2.degree. to 20.degree. F. The dewaxed oil-containing slurry is then fed to wax filters. This process if particularly useful for dewaxing wax-containing lubricating oil fractions and the like.
2. Description of the Prior Art
It is well known in the art to dewax wax-containing hydrocarbon oils, particularly the lube oil fractions of petroleum oil, in order to remove at least a portion of the wax therefrom to obtain a dewaxed oil of reduced cloud and pour points. The most common method of removing the wax or waxy constituents from waxy hydrocarbon oils is via the use of various solvent dewaxing processes. In solvent dewaxing processes the temperature of the wax-containing oil is lowered sufficiently to precipitate the wax therefrom as solid crystals of wax. At the same time, solvents are added to the waxy oil in order to improve the fluidity and reduce the viscosity thereof so that various filtration or centrifugation processes can be used to separate the solid particles of the wax from the dewaxed oil. Strong wax antisolvents (weak oil solvents) such as MEK are often added to decrease wax solubility in the oil/solvent mixture while strong oil solvents (weak wax antisolvents) such as MIBK or toluene are used to modify the solubility characteristics of the solvent so as to allow wax precipitation, while at the same time avoiding oil immiscibility at wax separation temperatures. Solvent dewaxing processes produce what is known as a pour-filter temperature spread. This is the temperature differential between the wax filtering temperature and the pour point of the dewaxed oil. This pour-filter temperature spread is greater when more non-polar hydrocarbon solvents are used than with more polar solvents such as ketones. Thus, an autorefrigerant dewaxing process employing propane can produce a pour-filter spread of 40.degree. F., which means that the wax filtration must be done at -40.degree. F. in order to produce a dewaxed oil having a pour point of 0.degree. F. When ketones or mixtures of ketone and aromatic solvents are used, the pour-filter spread may range from 0.degree. F. to 20.degree. F. depending on the oil and solvent used.
Both ketone and autorefrigerant dewaxing processes have certain advantages and disadvantages. Thus, although ketone dewaxing processes result in a lower pour-filter spread at the wax filtration temperature and although larger wax crystals can be grown in a ketone environment than in an autorefrigerant environment without dewaxing aid, ketones are relatively non-volatile compared to autorefrigerants, and, therefore, chilling of the solvent/oil mixture must be accomplished by either indirect means or by mixing cold ketone solvent with the oil. In the latter case, practical considerations limit the amount and temperature of cold ketone solvent that can be added and the temperature to which the solvent/oil mixture can be cooled. Therefore, some means of indirectly chilling the waxy slurry following the addition of solvent is required in all ketone dewaxing processes in order to bring the slurry down to the required wax filtration temperature. The most common method of indirect chilling is via the use of scraped surface chillers which are expensive and difficult to maintain. Also, the scraped surface chillers tend to damage the wax crystals by the shearing action of the scraper blades.
Conversely, wax crystals grown in an autorefrigerant environment, such as propane or propylene, are generally small which necessitates the use of costly dewaxing aids in order to achieve good filtration rates, although evaporation of the autorefrigerant enables one to reach the wax filtration temperature without the necessity of employing scraped-surface chillers or indirect heat exchangers following the solvent dewaxing operation. Additionally, it has been found necessary to employ batch chilling in autorefrigerant dewaxing processes in order to allow a gradual reduction in pressure. This prevents sudden flashing of the autorefrigerant at the point of pressure release, thereby avoiding sudden large temperature drops of the oil slurry (shock chilling), which would result in even smaller wax crystals and concomitant slower filter rates of the wax from the dewaxed oil.
In some ketone solvent dewaxing processes, the waxy oil and solvent, at a temperature above the cloud point of the oil, are mixed before being cooled. This solution is then cooled at a uniform, slow rate under conditions which avoid agitation of the solution as the wax precipitates out. In another method, ketone dewaxing solvent is added to the oil at several points along a chilling apparatus, but the waxy oil is first chilled without solvent until some wax crystallization has occurred and the mixture has thickened considerably, after which a first increment of solvent, at the temperature of the oil, is introduced in order to maintain fluidity. Cooling continues, more wax is precipitated out and a second increment of solvent, at the temperature of the mixture, is added to maintain fluidity. This process is repeated until a temperature typically ranging from about 30.degree. F. to 60.degree. F. is reached, at which point an additional amount of solvent at the same temperature as the mixture is added in order to reduce the viscosity of the mixture which is further chilled in scraped-surface chillers to the desired filtration temperature. In these processes, if the solvent is introduced at a temperature lower than that of the oil or oil/solvent mixture, shock chilling occurs resulting in the formation of small and/or acicula shaped wax crystals with attendant poor filter rate.
It is now well known that the adverse shock chilling effect can be overcome by introducing the waxy oil into an elongated, staged cooling zone or tower at a temperature above its cloud point and incrementally introducing cold dewaxing solvent into said zone, along a plurality of points or stages therein, while maintaining a high degree of agitation in said stages, so as to effect substantially instantaneous mixing of the solvent and wax/oil mixture as they progress through said zone. The basic concept of this commercially successful process is disclosed in U.S. Pat. No. 3,773,650, the disclosures of which are incorporated herein by reference and shall hereinafter be referred to as DILCHILL* dewaxing process. FNT *Registered service mark of Exxon Research and Engineering Co.
Commercially successful processes employing autorefrigerative cooling, wherein the waxy oil is mixed with a liquid autorefrigerant which is permitted to evaporate thereby cooling the oil by the latent heat of evaporation, are batch or semi-batch operations. This mixture of liquid autorefrigerant and oil are introduced into an expansion chamber wherein the pressure is slowly reduced to achieve controlled evaporation of the autorefrigerant and controlled cooling of the oil, thus avoiding the shock chilling which would result if the autorefrigerant were allowed to flash off. However, batch processes are cumbersome, difficult to operate and energy inefficient.
A number of attempts have been made to develop a continuous autorefrigerant process for dewaxing oils, including combinations of ketone/autorefrigerant processes. Thus, U.S. Pat. No. 3,549,513 discloses an autorefrigerative batch dewaxing process that is described as continuous but which really operates via the sequential use of a multiple number of batch chillers or expansion chambers. Waxy oil is diluted with an aromatic/ketone solvent mixture and with liquid autorefrigerant and cooling is achieved by controlled evaporation of the autorefrigerant by reducing the pressure in each batch chamber in a manner such that the autorefrigerant evaporates at a controlled rate. U.S. Pat. No. 3,658,688 discloses an autorefrigerant dewaxing process wherein a portion of the wax is precipitated from the oil in a DILCHILL dewaxing tower wherein the cooling occurs by the injection of cold autorefrigerant into the tower to produce a waxy slurry, followed by autorefrigerative cooling of the slurry in batch chillers. U.S. Pat. No. 2,202,542 suggests a continuous autorefrigerant dewaxing process wherein a waxy oil above its cloud point is premixed with warm, liquid propane. This mixture is introduced into a multi-staged cooling tower and liquid CO.sub.2 is injected into each stage out of direct contact with the oil. This patent emphasizes the point that the liquid CO.sub.2 must be introduced into each stage out of direct contact with the oil in the tower in order to avoid shock chilling. However, this is impractical because the vapor loads on the tower would be far in excess of what could be accommodated in a reasonably sized commercial tower. Also, refrigeration requirements are three times those normally needed and conditions for nucleation and growth of wax crystals are poor. U.S. Pat. No. 3,720,599 discloses a continuous process for dewaxing a waxy petroleum oil stock wherein the oil is premixed with acetone. This mixture is then introduced into a horizontal, elongated chilling vessel containing a plurality of stages operating at different pressures, with the pressure in each stage controlled by a back pressure regulator on each stage. Liquid autorefrigerant is introduced into the stages along the length of the chilling vessel while maintaining a high degree of agitation therein to avoid shock chilling. The autorefrigerant is partially evaporated in each stage, with the amount of evaporation being controlled by the pressure in each stage. Unfortunately, there are problems which currently preclude commercialization of this process, not the least of which is a practical, efficient way of getting the slurry to flow from stage to stage without plugging up the entire apparatus with wax or without multiple transfer pumps which would be expensive and would also tend to destroy the wax crystal structure. Another disadvantage entails the impracticality of providing separately driven agitators for each stage and the mechanical difficulties associated with a common horizontal drive shaft. Additionally, U.S. Pat. No. 3,720,599 provides for the nucleation and initial growth of wax to occur in the presence of substantial amounts (i.e., &gt;25%) of autorefrigerant solvent, which, in the absence of dewaxing aid, has been found to produce wax crystals inferior to those produced when nucleation occurs by chilling in the presence of ketones or ketone/aromatic solvents followed by autorefrigeration. For example, when mixtures of ketone and high percentages (&gt;40%) of propylene were used in the DILCHILL dewaxing process, a distillate oil/wax slurry was produced which filtered very poorly.
It would be an improvement to the art if one could combine both ketone and autorefrigerant solvent dewaxing processes into a continuous process and in such a manner so as to carefully form the wax nuclei and begin crystal growth in a substantially non-autorefrigerant solvent environment such as ketone, to achieve large, stable, spherical crystals without the use of dewaxing aid and then further precipitate additional wax without destroying the spheres via direct contact with an evaporating autorefrigerant, thereby avoiding the need for scraped surface chillers following the ketone dewaxing step.